Digital optical telecommunication links between a plurality of satellites or between one satellite and operational or experimental earth stations conventionally use technologies operating at a wavelength of 0.8 μm or 1.06 μm for the optical communication signal. These links use a single modulated and encoded optical carrier to transmit the digital data at a medium rate or at a high rate, these rates being able to be in the region of several tens of Mbps (Megabits per second) to several Gbps (Gigabits per second) according to the technology used. On transmission, the optical carrier is transmitted by a laser beam telescope, the wavefront of which is plane on entry into the atmosphere. However, the passage through turbulences in the atmosphere, in the case of a link between a satellite and an earth station, causes distortions in the wavefront of the laser beam. These distortions cause three phenomena in the telescope on reception. The first phenomenon, called scintillation, is a variation in the energy received on the pupil of the telescope. The second phenomenon is the random displacement of the laser beam (“beam wander”) which causes a variation in the angle of arrival of the laser beam on the pupil of the telescope, more or less effectively compensated by the fine pointing system of the telescope. The third phenomenon is the phase and amplitude variation of the optical signal (“wavefront error”) on the surface of the pupil of the telescope which causes aberrations in the focal plane of the telescope which renders the coupling between the focus of the telescope and the input optical fibre of the receiver less effective. These three phenomena combine with one another to produce rapid and random variations in the optical signal level in the receiver input optical fibre. These power level variations manifest themselves as periods during which the signal is very weak and can no longer be detected and demodulated by the receiver. These periods are periods of fading of the digital optical link, these fading periods being both long, for example their duration is in the region of several hundreds of milliseconds, and deep, for example greater than minus ten decibels. During each fading period, the transmission of the binary data is interrupted, causing errors over a plurality of bits or an erasure of the data.
In order to correct these transmission errors, it is known to use error-correcting codes, however, the error-correcting codes can only correct individual, isolated errors and cannot correct long sequences of consecutive errors.
Resistance to the fading of the signal transmitted on the digital optical link can be obtained by time-interleaving of the binary data in such a way as to distribute the errors in time. This time-interleaving is applied in addition to the error-correcting codes, thereby ensuring the integrity of the transmitted data.
An interleaver is a device that splits up long periods of errors which have occurred during the signal attenuation period and which have been generated by the internal decoder, by distributing these errors in time in such a way that these errors can easily be corrected by the error-correcting code. Different types of interleaving exist, such as block interleavers, convolutional interleavers, random interleavers.
The duration of the time-interleaving must be of the same order of magnitude as the duration of the fading of the optical link. The duration of the fading being in the order of several hundreds of milliseconds (ms), to implement an optical link with a high data transmission rate, said rate being between several tens and several hundreds of Gbps, the implementation of the interleaving and encoding is difficult, or even impossible, with current digital electronics technologies. In particular, the memory sizes of known interleavers are insufficient to be able to store the data during long periods, and the processing rates of the encoders and decoders are too slow.
In order to increase the transmission rates, it is known to use a technology operating in the infrared band in wavelengths around 1550 nm. Document U.S. Pat. No. 7,277,644 describes an error-correcting method for optical transmission in free space operating in the infrared band around the 1550 nm (nanometer) wavelength. The method consists in copying the same binary signal incoming on a plurality of parallel channels and in transmitting the different copies on a plurality of optical links of different carriers and in selecting only the validated optical links in order to average the effects due to the scintillation phenomenon. However, the processing of the binary data on each link is carried out at the same rate as the incoming signal data, which does not allow the rate of processing of the binary data by the encoders and the interleavers to be reduced, the data at a very high rate above several Gbps to be processed, or the problem of scintillations with a duration greater than around ten milliseconds to be solved. Furthermore, this method, which uses the principle of redundancy of the data which are copied and processed on a plurality of parallel links, has the disadvantage of being costly in terms of power, since it requires the same information to be processed at the same rate as many times as there are different optical links.